In many wireless communication systems, a power amplifier (PA) used in the transmitter is required to be very linear. High linearity is required to prevent leakage of interfering signal energy between the intended channels. In addition, it has to allow for simultaneous amplification of many radio channels or frequencies spread across a fairly wide bandwidth. In order to reduce power consumption and need for cooling, the amplifier also has to have a high efficiency.
It is difficult to provide linear RF currents with high efficiency for practical high-power RF transistors. Practical high-power RF transistors have a transconductance (output current per input voltage) that is not constant, but changes with the input node voltage. A conventional way of getting a linear RF current response with high DC-to-RF conversion efficiency is to bias the device in a so-called class B operation (for class definitions, see e.g. [1]), where an output current is provided during half of each RF cycle. The biasing is performed by providing a DC voltage offset at the input node, such as the gate of a field-effect transistor (FET) device or the base of a bipolar transistor. If the transconductance of a class B device ideally is constant for positive input signal voltages and zero for negative input signal voltages, the RF output current is linear and the maximum efficiency is 78.5% as discussed e.g. in [2]. Since the DC current consumption is proportional to the RF current, the power efficiency of a class B amplifier is proportional to the output amplitude, provided that the drain or collector supply voltage is kept constant.
In real RF transistors, the transconductance may differ significantly from constant. It can e.g. be a substantially linear function of the input node voltage, or a mixture of a mostly linear part and a mostly constant part. This behavior is normally referred to as quasi-linear. In the case of a purely linear transconductance, the output signal amplitude is proportional to the input signal squared for a class B biased amplifier. Such response is sometimes referred to as a parabolic transfer function.
Since RF power transistors typically are relatively expensive, there is also an incentive to get the most possible output power from them. An important factor in getting the most output power from a transistor is to have the right load impedance at the fundamental frequency, i.e. the desired RF. Practical transistors have limits to both the maximum output current and the maximum output node voltage. To obtain the maximum output power from the device, it is therefore important that these limits are reached simultaneously.
Besides the above-described class B biasing, there are also other biasing possibilities. An amplifier biased in class A is always giving an output current. An amplifier in class AB is biased between class A and class B. A class A amplifier gives the highest output RF power for a given limitation in the maximum peak current, when most devices with linear or quasi-linear transconductance are used. For a device with a constant transconductance, biasing in class A and class B gives the same out-put power, and class AB slightly more. With a tuned load, i.e. all voltage harmonics at the output node perfectly short-circuited or “terminated” by a reactance network, and a device with a linear transconductance, class B operation gives 0.7 dB less output power than class A, see e.g. [2]. By terminating the voltage harmonics, in particular the second harmonic, at the output node makes it possible to use a larger voltage swing of the fundamental frequency. However, the harmonic termination is seldom perfect in practice. Since the harmonic content of the class B current waveform generally is greater than that of class A, the difference in output power can be even larger than 0.7 dB.
Biasing in class C means that the RF current is on during less than half of each RF cycle. This biasing gives generally the highest efficiency, but gives highly nonlinear response and considerably less peak output than either class A or B. In order to maximize the peak output, class A biasing is required, but in order to maximize the efficiency, class C is to prefer. In addition to this linearity considerations have to be taken.
A simple way of getting a linear response from a device with linear transconductance is to bias it in class A. This gives, however a very low average efficiency if the peak-to-average power ratio is high, due to the high quiescent DC supply current. A quasi-linear device can be biased in class AB, and have a substantially linear response for less quiescent current. The average efficiency is therefore better than for devices with linear transconductance biased in class A, but worse than devices with constant transconductance biased in class B. In order to achieve a better linearity, the bias level can be controlled adaptively. Such bias control is generally much slower than the signal variations. Solutions using adaptive bias levels are discussed e.g. in [3-7].
In many cases, bias level adjustments, i.e. choosing the best static bias, are not enough to obtain sufficient linearity. Further linearization methods must typically be employed, see e.g. [5-7]. One often used method for providing high efficiency and wide bandwidth is pre-distortion, used e.g. in [5, 7]. The input signal to the transistor is nonlinearly pre-compensated to counteract the distorting behavior of the transistor itself. In some cases, the linearizing performance of the pre-distortion method is even sufficient to allow for the device to be biased closer to class B, which normally implies a higher degree of non-linearity. However, the quiescent current is reduced and the average efficiency is increased.
Another way of getting a more linear response from a practical device is to use a dynamic bias. This means that the bias level is varied substantially at the same speed as the amplitude of the amplified signal. Examples of such systems are found in [8-13]. In [8], such technique has been used e.g. for improving the power-added efficiency of class A amplifiers with low gain. A constant gain device is used, which can be kept in class A with a bias level proportional to the signal amplitude.
Two different dynamic biasing schemes optimizing either efficiency or intermodulation distortion are compared in [9]. In [10], it is proposed to take care of extra non-linearity introduced by dynamic biasing by applying pre-distortion. Using dynamic biasing to provide constant gain (and therefore linear amplification) with higher efficiency than class A operation is described in [11], and with the addition of feedback for controlling the bias level, in [12]. In [11], the device is biased in class AB at low amplitudes, and ends up in class B at high amplitudes, and in [12] the device is biased in class AB, and the bias variations are only used for small corrections by feedback to the momentary gain. Phase distortion is handled by a separate feedback loop. Constant gain is also the objective in [13], in which a dynamic bias scheme is added onto a Doherty high-efficiency amplifier.
[14] discloses a bias circuitry configured to adjust bias current to one or more power amplifier stages based upon the level of the RF signal to be amplified. The bias circuitry eliminates excessive quiescent bias currents that prior biasing technique required to ensure linear operation by automatically increasing bias currents only as needed based on the effective magnitude of the RF signal to be amplified.
In [15] a power transistor circuitry is disclosed, which for increasing the gain of an amplified RF signal has a power transistor, a voltage bias circuit having a peak detector, and a voltage source. A summer sums the outputs of the peak detector and the voltage source to increase the bias and provide an increased gain upon detection of an RF signal peak.